Prostate cancer is the most frequently diagnosed cancer and second leading cause of cancer death in men. Some 45,000 men die annually of this disease. Only lung cancer has a higher mortality. The chance of a man developing invasive prostate cancer during his lifetime is 1 in 6. At the age of 50, a man has a greater than 40% chance of developing prostate cancer and nearly a 3% chance of dying from this disease. While some advances in the treatment of locally confined tumors have been achieved, prostate cancer is incurable once it has metastasized. Patients with metastatic prostate cancer are treated by hormonal ablation therapy, but with only short-term success. Eventually, these patients develop an androgen-refractory state leading to disease progression and death.
A continuing and fundamental problem in the management of prostate cancer is the absence of reliable diagnostic and prognostic markers capable of accurately detecting early-stage localized tumors and/or predicting disease susceptibility and progression. Early detection and diagnosis of prostate cancer currently relies on digital rectal examination (DRE), prostate specific antigen (PSA) measurements, transrectal ultrasonography (TRUS), and transrectal needle biopsy (TRNB). Serum PSA measurements in combination with DRE represent the leading diagnostic approach at present. However, this approach has major limitations which have fueled intensive research into finding better diagnostic markers of this disease. A number of markers have been identified, and at least one, PSA, is in widespread clinical use. However, ideal prostate tumor markers have been extremely elusive and no marker has yet proven reliable for predicting progression of the disease. Thus, there is a need for more reliable and informative diagnostic and prognostic methods in the management of prostate cancer.
In addition, there is also great interest in identifying prostate-specific proteins that could be appropriate as therapeutic targets, as there is no effective treatment for patients who develop recurrent disease or who have been diagnosed with metastatic disease. Although hormone ablation therapy can palliate these patients, the majority inevitably progress to develop incurable, androgen-independent disease (Lalani et al., 1997, Cancer Metastasis Rev. 16: 29-66).
PSA is the most widely used tumor marker for screening, diagnosis, and monitoring prostate cancer today. In particular, several immunoassays for the detection of serum PSA are in widespread clinical use. Recently, a reverse transcriptase-polymerase chain reaction (RT-PCR) assay for PSA mRNA in serum has been developed. However, PSA is not a disease-specific marker, as elevated levels of PSA are detectable in a large percentage of patients with BPH and prostatitis (25-86%)(Gao et al., 1997, Prostate 31: 264-281), as well as in other nonmalignant disorders and in some normal men, a factor which significantly limits the diagnostic specificity of this marker. For example, elevations in serum PSA of between 4 to 10 ng/ml are observed in BPH, and even higher values are observed in prostatitis, particularly acute prostatitis. BPH is an extremely common condition in men. Further confusing the situation is the fact that serum PSA elevations may be observed without any indication of disease from DRE, and vice-versa. Moreover, it is now recognized that PSA is not prostate-specific and has a variety of complex biological activities (See e.g. Fortier et al., J. Natl. Cancer Inst. 1999, 91(19):1635-40).
Various methods designed to improve the specificity of PSA-based detection have been described, such as measuring PSA density and the ratio of free vs. complexed PSA. However, none of these methodologies have been able to reproducibly distinguish benign from malignant prostate disease. In addition, PSA diagnostics have sensitivities of between 57-79% (Cupp & Osterling, 1993, Mayo Clin Proc 68:297-306), and thus miss identifying prostate cancer in a significant population of men with the disease.
Prostate-Specific Membrane Antigen (PSMA) is a recently described cell surface marker of prostate cancer which has been the subject of various studies evaluating its use as a diagnostic and therapeutic marker. PSMA expression is largely restricted to prostate tissues, but detectable levels of PSMA mRNA have been observed in brain, salivary gland, small intestine, and renal cell carcinoma (Israeli et al., 1993, Cancer Res 53: 227-230). PSMA protein is highly expressed in most primary and metastatic prostate cancers, but is also expressed in most intraepithelial neoplasia specimens (Gao et al., supra). Preliminary results using an Indium-111 labeled, anti-PSMA monoclonal antibody to image recurrent prostate cancer show some promise (Sodee et al., 1996, Clin Nuc Med 21: 759-766). PSMA is a hormone dependent antigen requiring the presence of functional androgen receptor. Since not all prostate cancer cells express androgen receptor, the clinical utility of PSMA as a therapeutic target may be inherently limited. Clinical trials designed to examine the effectiveness of PSMA immunotherapy are also underway.
Prostate Stem Cell Antigen (PSCA) is another recently described cell surface marker of prostate cancer (Reiter et al., 1998, Proc. Natl. Acad. Sci. USA 95: 1735-1740). PSCA expression has been shown to be predominantly prostate specific and widely over-expressed across all stages of prostate cancer, including high grade prostatic intraepithelial neoplasia (PIN), androgen-dependent and androgen-independent prostate tumors. The PSCA gene has been mapped to chromosome 8q24.2, a region of allelic gain in more than 80% of prostate cancers. PSCA shows promise as a diagnostic and therapeutic target in view of its cell surface location, prostate specificity, and greatly unregulated expression in prostate cancer cells.
Progress in the identification of specific markers has been slow due to a lack of experimental animal model systems that recapitulate clinical disease. Attempted solutions to this problem have included the generation of prostate cancer cell lines (Horoszewicz et al., 1983, Cancer Res. 43, 1809) and prostate cancer xenografts (Pretlow et al., 1991, Cancer Res. 51, 3814; van Weerden et al., 1996, Am. J. Pathol. 149, 1055; Klein et al., 1997, Nature Med. 3, 402). However, these approaches have met with limited success. For example, xenografts have generally produced low long-term survival rates. In addition, none of the most widely used human prostate cancer cell lines—PC-3, DU-145, and LNCaP—have been shown to reproducibly give rise to osteoblastic lesions typical of prostate cancer. A further limitation of the DU-145 and PC-3 cell lines is that these cells do not express prostate specific antigen (PSA) or androgen receptor (AR) (Kaighn et al., 1979, Invest. Urol. 17: 16-23; Gleave et al., 1992, Cancer Res. 52: 1598-1605), questioning their relevance to clinical prostate cancer. The LNCaP cell line is androgen responsive and expresses PSA, but contains a mutation in the androgen receptor which alters ligand specificity.
Recently, however, a series of prostate cancer xenografts (derived from patient tumors) demonstrating genetic and phenotypic characteristics closely paralleling the human clinical situation have been described (Klein et al., 1997, Nature Med. 3: 402). These LAPC (Los Angeles Prostate Cancer) xenografts have survived passage in severe combined immune deficient (SCID) mice for longer than one year. The LAPC-4 xenograft model systems has the capacity to mimic the transition from androgen dependence to androgen independence and the development of metastatic lesions (Klein et al., 1997, supra). LAPC-4 tumors regress in male mice after castration, but re-grow within 2-3 months as androgen independent tumors. Both androgen dependent (AD) and androgen independent (AI) LAPC-4 xenograft tumors express equal levels of the prostate specific markers PSA, PSMA and PSCA (prostate stem cell antigen), which was identified using representational difference analysis of cDNAs derived from the AD and AI variants of the LAPC-4 xenograft.